The objective of our research is to contribute towards a description of the molecular biology of replication of Rous sarcoma virus, an avian RNA tumor virus. Towards this goal, we will utilize mutants of Rous sarcoma virus to study initiation of infection leading to an integrated provirus, maintenance of the integrated provirus, synthesis and assembly of virion proteins and RNAs, and the nature of src and its products. For this purpose we have isolated novel mutants for each gene of Rous sarcoma virus, including (a) two temperature sensitive mutants which appear to map in gag; (b) six temperature sensitive mutants and one deletion mutant, representing five different phenotypes, which appear to map in pol; (c) two temperature sensitive mutants and one deletion mutant mapping in env; and (d) one insertion mutant, with apparently wild type functions, which contains an insertion of sequences into src. These mutants will be analyzed by standard biological procedures for the study of avian tumor viruses, combined with immunoprecipitation procedures for the analysis of viral proteins in lysates of infected cells and hybridization techniques for the analysis of viral DNA and RNA in infected cells.